No. 3-] THE VERTEBRATE HEAD. 513 



III. Descriptions of Stages of Acanthias. 



The earliest stage in which I have detected this metameric 

 segmentation is represented in PI. XXVII, Fig. 25. This is 

 an age somewhere between Balfour's stages B and C. It is, 

 in reality, the youngest embryo of Squalus to which I have 

 had access since I began to observe especially the metameric 

 segments in that animal. Whether or not they occur in still 

 younger embryos I do not know, but they are already clearly 

 defined in the stage referred to, and it is reasonable to suppose 

 that they may be seen in still earlier stages. 



The axial embryo (Fig. 25) is just fairly established, and 

 has reached a length of ixV™rn- The head-end is already 

 wider than the rest of the embryo. It has begun to show that 

 tendency to broaden that is characteristic of the head-end of 

 the embryo. The gastrular cavity is broad, and extends to 

 the extreme anterior end of the embryo. In the figure it is 

 seen even protruding beyond the head-plate. The primitive 

 furrow, that has often been confused with the neural groove 

 in these Elasmobranchs, is broadened at its anterior end. Fig. 

 63, PI. XXIX, is a sketch of a horizontal section of this 

 embryo to show the general appearance of the metameres in 

 section. 



The segmental divisions in this embryo extend from the 

 anterior end backwards along the margins of the axial part 

 of the embryo, and out into the non-axial part or embryonic 

 rim. There are seven or eight pairs of these segments in the 

 embryo, and as many more, directly continuous with them, in 

 the embryonic rim. The latter is segmented to the points 

 where it is broken from the rest of the blastoderm. Whether 

 or not these segments extend further into the blastodermic 

 rim, I am unable to say. 



The segments are most clearly defined along the inner mar- 

 gin of the embryonic rim, and extend more faintly across it. 

 In the axial part of the embryo they are not in such a favor- 

 able position to be observed from above — they are on the 

 rounded margins ; but if the embryo be rolled into such a posi- 

 tion that the margin is brought into view, its division into 



